Neutron Properties and Definitions
Lawrence Heilbronn
This document is meant as a supplement and guide to the slides currently used for the neutron
lectures in the NASA summer school’s sessions on radiation physics. Those slides may be
found at http://three.usra.edu/articles/NeutronPropertiesDefinitions.swf. Because not all of the
slides will be shown in this document, it is recommended that a copy of the slides be available
as the reader goes through this text.
Neutron Properties
There are basic properties of the neutron that must be presented in order to understand how
and why neutrons are unique relative to the other components of the radiation environment in
space. One of the most important properties is the lifetime of the neutron. A free neutron has a
half life of approximately 15 minutes (the neutron decays via beta decay to a proton and an
antineutrino). What this effectively means is that there are no neutrons in the primary Galactic
Cosmic Ray (GCR) environment. By the time GCR enters our solar system, any neutron
created at the source of the GCR has decayed away. There are some neutrons emitted from
the sun that do live long enough to reach the vicinity of Earth, but those are very few in number
compared to the other radiations emitted from the sun.
If there are essentially no neutrons in the primary radiation environment in space, why do
neutron monitors on the ISS and spacecraft indicate a significant number of neutrons inside
those environments? Those neutrons are created by nuclear interactions between the primary
GCR and any material it comes in contact with, including the spacecraft hull, other structural
materials, and humans. As the amount of material increases, the number of generated
neutrons also increases.
As one increases the amount of material and shielding in a spacecraft, the dose from the
charged particle component of GCR decreases because those particles slow down (see
“stopping power” discussion in other parts of the course) and stop or break up due to nuclear
interactions. However, compared to charged particles, neutrons are much more highly
penetrating and can go through shielding without interacting because they have no charge. As
a result, neutrons can become more significant in terms of their contribution to the total dose
and effective dose behind thick shielding in space.
The dose and subsequent biological damage delivered by neutrons depends greatly on the
energy of the neutron. On Earth, radioactive sources of neutrons generate neutrons between 0
and 10 MeV. In space, the energy spectrum of neutrons is much different, with neutron
energies going well beyond 10 MeV, up to several TeV. The presence of high energy neutrons
poses problems not only for dosimetry and monitoring, but also for radiobiologists seeking to
understand the biological effects from high-energy neutrons.
Neutron Energy Classification
Neutron properties and definitions (supplement). Heilbronn L. https://three.jsc.nasa.gov/articles/Heilbronn_Neutron_Supplement.pdf. Date posted: 07-09-2015.
The followings terms used to describe neutron energy ranges were created primarily to
distinguish how neutrons interact with materials used for applications utilizing fission, such as
nuclear power, weapons development, and isotope production. They are relevant for the
discussion of biological effects from neutrons, as well, because again how neutrons interact in
the body depends very much on their energies.
Note that the energy ranges shown in the definitions below are not absolute, but are “ballpark”
figures meant to convey information on the general range of energies.
1. Cold neutrons – energies below thermal energies (see next definition), typically
corresponding to meV and sub meV energies, i.e., from 0 to 0.025 eV. Cold neutrons
are used in a variety of applications, including studies on the structure of bio molecules.
Both scattering and absorption reactions can occur at these energies (scattering and
absorption are defined later), although for most atoms that compose biological material,
the dominant reaction is scattering.
2. Thermal neutrons – the energy of neutrons that are in equilibrium with the motion of the
atoms that make up the medium in which the neutron is found. Neutrons colliding with
atomic nuclei either pick up energy if they are moving slower than the colliding nucleus,
or lose energy if they are moving faster. This constant slowing down and picking up of
energy by free neutrons leads to a distribution of neutron energies centered about the
most likely energy (thermal energy). The distribution is called a Maxwell – Boltzmann
distribution, which is shown in the plot below. For biological materials, scattering is the
dominant reaction, although absorption of thermal neutrons by hydrogen and nitrogen is
also present and can deliver dose to the biological material.
3. Epithermal neutrons – energies between thermal (~.025 eV) and a few hundred eV.
This represents the transition region between thermal and slow neutrons where
Neutron properties and definitions (supplement). Heilbronn L. https://three.jsc.nasa.gov/articles/Heilbronn_Neutron_Supplement.pdf. Date posted: 07-09-2015.
“resonances” present in the nuclei that interact with the neutron begin to present
themselves. Resonances are very important in the next energy range, slow neutrons.
4. Slow neutrons – generally have energies between 100’s of eV to 0.5 or 1 MeV. In this
energy range, many of the nuclei that the neutron interacts with have nuclear structure
within the combination of neutrons and protons that make up the nucleus, and that
structure leads to an enhanced probability of interaction between the neutron and
nucleus. The atoms than make up most biological materials – H, C, N and O – don’t
have the significant resonances that heavier atoms such as the actinides have.
5. Fast neutrons – generally between 0.5 and 10 - 20 MeV. These are the energies of
neutrons emitted by fission sources. This represents the upper limit of where most of
the research on neutron radiobiology and fundamental neutron interaction cross sections
has been done. This is not to say that there has been no research above these
energies, it’s just that the amount of information and data at energies above 20 MeV is
not as well established as the data below.
6. High energy neutrons – above 20 MeV. A significant fraction of the dose and effective
dose from neutrons in space is delivered by high energy neutrons and represents the
region of greatest uncertainty in the biological effects from neutrons in space.
Neutron Interactions
Neutron interact via two main reaction mechanisms: scattering and absorption. Both types of
interactions can occur at any energy, but in general scattering reactions dominate once the
neutron energy is above a few hundred keV. Scattering can also dominate at energies below a
few hundred keV, but at those lower energies absorption can become significant, especially at
thermal and slow energies.
Most terrestrial applications of neutron radiobiology have dealt with fission neutrons with
energies above 0.5 MeV. At those energies, scattering is the main mechanism for delivering
dose is neutron scattering, and in the human body, the main interaction is neutron-hydrogen
elastic scattering.
A. Scattering interactions
In a scattering interaction, the neutron remains free after the interaction, but loses energy by
transferring energy to the nucleus it strikes. An elastic scattering interaction is one in which the
total energy, momentum, and kinetic energy are conserved. An inelastic interaction is one in
which only total energy and momentum are conserved. Typically, in an inelastic interaction, the
energy transferred to the struck nucleus can break up the nucleus into two or more pieces or
leave the nucleus in an excited state. In all scattering interactions, the neutron delivers dose via
indirect ionization, meaning that first the neutron transfers energy to a charged recoil particle
(such as a nucleus, proton, deuteron, etc..), and then that charged recoil particle deposits its
energy, leading to possible biological and material effects.
Neutron properties and definitions (supplement). Heilbronn L. https://three.jsc.nasa.gov/articles/Heilbronn_Neutron_Supplement.pdf. Date posted: 07-09-2015.
The power point slides accompanying this document show crude animations of elastic and
inelastic neutron interactions with various nuclei. Note that in addition to the charged particle
recoils that are produced and can deliver dose to the surrounding material, uncharged radiation
such as gamma rays and scattered neutrons can also be produced. The important distinction
between charged and uncharged secondary radiation is that the uncharged radiation can be
deeply penetrating through the material, whereas the charged secondary particles have much
shorter ranges in the material and are not considered deeply penetrating. These charged
particles, however, typically have high values of Linear Energy Transfer (LET, see previous
lectures), and as such have high associated quality factor (Q) values.
In the human body, fission energy neutrons interact primarily via elastic scattering with tissue
components (H, C, N and O). There are well defined kinematical limits for the energies of the
scattered recoil nuclei, which are shown in terms of the maximum fraction of the incoming
neutron’s energy given to the recoil nucleus in the table below.
Nucleus Maximum fraction of neutron energy transferred to recoil nucleus 1H 1.000 12C 0.284 14N 0.249 16O 0.221
For example, if a 5 MeV neutron elastically scattered off of a 12C nucleus, the maximum energy
the recoil 12C could have is:
(max. fraction) x (neutron energy) = (0.284) x (5 MeV) = 1.42 MeV.
Note that this isn’t the only scattered energy the recoil 12C can have, it’s the maximum. The
recoil 12C can have any energy between zero and the maximum, with every energy in that range
equally likely. Thus, on average, the recoil nucleus has ½ of the maximum recoil energy. In the
example used above, for a 5-MeV neutron scattering off of 12C, the average energy the 12C has
is:
(1/2) x (max recoil energy) = (½) x (1.42 MeV) = 0.71 MeV.
Once the neutron scatters, it can continue on and interact again and again until the neutron
stops and is absorbed, or punches through the material. Generally, in the human body, the
neutron scatters just once, with a small probability of additional scattering. The “First Collision
Dose”, the dose delivered to the human body by the first scatterings between neutrons and
tissue, accounts for 80 to 90 percent of the total dose delivered to the body from neutrons.
The first collision dose can be calculated with the following equation:
𝐷 =Φ𝑁𝜎𝑄𝑎𝑣𝑒𝑟𝑎𝑔𝑒
𝜌
Where D = dose (energy deposited per unit mass)
Φ = neutron fluence (number of neutrons per unit area)
N = # of nuclei per cm3
Neutron properties and definitions (supplement). Heilbronn L. https://three.jsc.nasa.gov/articles/Heilbronn_Neutron_Supplement.pdf. Date posted: 07-09-2015.
σ = interaction cross section (unit of cm2, related to the probability of an interaction)
ρ = density of the material (i.e., density of tissue, water,…)
Qaverage = Average energy transferred to the recoil nucleus (= ½ max energy transferred)
As an example, the following is a calculation of the first collision dose from a fluence of 5-MeV
neutrons scattering off of the hydrogen in the human body. The cross section for elastic
scattering of 5-MeV neutrons off of protons (hydrogen nuclei) is approximately 1.6 barns (=
1.6x10-24 cm2). Since the maximum energy transfer to a proton from a 5-MeV neutron is 5 MeV,
the average energy transferred to the proton is 2.5 MeV. The number of hydrogen nuclei per
cm3 in tissue is 5.98x1022. The density of tissue varies, but in this case a value of 0.92 g/cm3 is
used. Assuming that the neutron flux is 108 neutron/cm2, the dose is:
𝐷 =(108𝑐𝑚−2)(5.98𝑥1022𝑐𝑚−3)(1.6𝑥10−24𝑐𝑚2)(2.5 𝑀𝑒𝑉)
0.92 𝑔 𝑐𝑚−3= 2.6𝑥107
𝑀𝑒𝑉
𝑔
In the SI unit of dose from radiation exposure,
𝐷 = 2.6𝑥107𝑀𝑒𝑉
𝑔×
1.6𝑥10−13𝐽
𝑀𝑒𝑉×
1000 𝑔
𝑘𝑔= 0.00416
𝐽
𝑘𝑔= 0.00416 𝐺𝑦
B. Absorption interactions
In an absorption reaction, neutrons are absorbed by the nucleus, and as a result can deposit a
significant amount of energy into that nucleus. The combined neutron + nucleus system,
referred to as a compound nucleus, is often in an excited state and can de-excite through a
number of different pathways, including the emission of charged secondary particles and
uncharged (neutrons and gammas) radiation. As with scattering interactions, the charged
secondaries have high LET and high Q values. The following figures show the basic steps in
one such absorption reaction, neutron absorption in a 10B nucleus, leading to the emission of an
alpha and a 7Li particle.
Figure 1a: An incoming thermal neutron strikes a 10B nucleus.
Neutron properties and definitions (supplement). Heilbronn L. https://three.jsc.nasa.gov/articles/Heilbronn_Neutron_Supplement.pdf. Date posted: 07-09-2015.
Figure 1b: The neutron from fig. 1a is absorbed in the 10B nucleus, forming a compound 11B
nucleus.
Figure 1c: The compound 11B nucleus breaks up, emitting a 4He nucleus, 7Li nucleus, and a
gamma ray.
In these types of absorption reactions, energy is often released via the emission of charged
particle secondaries, such as the He and Li particles that are emitted in 10B neutron capture.
Typically, these charged secondary particles have very high LET values, and as a result deliver
a large dose in a very small volume, with a high accompanying quality factor Q due to the high
LET values.
To summarize neutron interactions, we’ve seen that neutrons interact via elastic scattering,
inelastic scattering, and absorption. All of these interactions are nuclear interactions
(interactions with atomic nuclei), and because of this the pattern of energy deposition by a
neutron in a material is much different than how an incoming charged particle deposits energy.
Whereas a charged particle deposits energy continuously through a material, a neutron deposits
energy randomly in confined areas in the material, in a stochastic manner. As a result, the
determination of the dose and dose equivalent delivered by neutrons in a material requires a
different approach than what you have learned about charged particle dosimetry, and that is the
subject of the next section.
Neutron properties and definitions (supplement). Heilbronn L. https://three.jsc.nasa.gov/articles/Heilbronn_Neutron_Supplement.pdf. Date posted: 07-09-2015.
Neutron Dosimetry Concepts
In the previous section we learned that a neutron can interact in a number of different ways in a
material, and that each interaction can emit a variety of particles over a wide range of energies.
Each interaction has an associated probability of that interaction, and as such it is possible to
average over all of the possible interactions to give an average energy released to secondary
charged particles per incoming neutron, as well as determine an average LET value. Note that
these average quantities are a function of the incoming neutron energy as well as the material it
goes through.
One average quantity that can be calculated is the KERMA (Kinetic Energy Released per unit
MAss), which described the average energy released to charged secondary particles per unit
mass. Note that KERMA will have the same units of dose (energy per unit mass), but is NOT
the dose due to one subtle difference: KERMA is energy transferred to charged particles per
unit mass, whereas dose is the energy absorbed from charged particles per unit mass. It is not
necessarily true that all of the energy given to a charged secondary particle will be absorbed in
the mass in which it is created.
The KERMA can be calculated with the following:
If all of the energy transferred to charged particles is absorbed in the medium (or if the energy
not absorbed is taken into account), then one can use KERMA values to determine the dose
delivered per unit fluence of neutrons. The table shown below gives one determination of the
dose per unit fluence of neutrons, which in this case assumes an isotropic fluence of neutrons
incident upon an adult male.
Neutron energy Fluence for 1 cGy
thermal 2.9 x 109
5 keV 2.1 x 109
20 keV 2.0 x 109
100 keV 9.6 x 108
500 keV 4.3 x 108
1 MeV 2.7 x 108
Neutron properties and definitions (supplement). Heilbronn L. https://three.jsc.nasa.gov/articles/Heilbronn_Neutron_Supplement.pdf. Date posted: 07-09-2015.
5 MeV 1.8 x 108
10 MeV 1.6 x 108
20 MeV 9.0 x 107
50 MeV 5.2 x 107
100 MeV 4.2 x 107
200 MeV 2.6 x 107
500 MeV 1.1 x 107
1000 MeV 5.4 x 106
As mentioned above, in addition to calculating the average energy deposited by neutron
interactions, one can determine the average LET value per neutron interaction, and
consequently determine a radiation weighting factor associated with the dose delivered by
neutrons. The radiation weighting factor is similar to the quality factor Q in that the dose is
multiplied by a weighting factor to determine the equivalent biological effectiveness of that dose,
allowing for a more direct comparison of the different types of radiation that can deliver dose to
the body. In this case, multiplying the neutron dose by its radiation weighting factor yields the
Equivalent Dose (as opposed to Dose Equivalent when multiplying charged particle dose by the
quality factor Q). The figure below shows the old and new neutron radiation weighting factors
as a function of neutron energy. The new factors were proposed after a reassessment of the
dosimetry in the Nagasaki and Hiroshima nuclear weapon detonations.
Neutron properties and definitions (supplement). Heilbronn L. https://three.jsc.nasa.gov/articles/Heilbronn_Neutron_Supplement.pdf. Date posted: 07-09-2015.
Figure 2. Neutron radiation weighting factors.
The physics behind these neutron dosimetric concepts is continually being improved with
information gathered from new measurements and better modeling. In much the same manner,
understanding the radiobiological effects of neutron irradiation is a field that needs more data
and is open to wide range of possible experiments. One critical requirement for neutron
radiobiology experiments is a reliable dosimetry system that can deliver accurate information on
the dose, neutron energy (or energies), and neutron field shape. The next section covers some
of the standard neutron dosimetry methods currently being used.
Neutron Dosimetry Methods and Instrumentation
When performing neutron radiobiology experiments, the neutron dosimetry system will most
likely be required to deliver all or some of the following:
1. Monitor and determine the dose on target
2. Control the shape and size of the neutron beam (or neutron field)
3. Determine the neutron energy spectrum
4. Determine the level of contaminants in the neutron field (if any)
5. Minimize the amount of room scattered neutron striking the target
Neutron properties and definitions (supplement). Heilbronn L. https://three.jsc.nasa.gov/articles/Heilbronn_Neutron_Supplement.pdf. Date posted: 07-09-2015.
Most important of all of these is #1, the ability to monitor and determine the dose on target. The
monitoring system should be able to:
a. Have a real time response
b. Distinguish neutrons from possible contaminants, such as gamma rays
c. Respond to a wide range of neutron energies, and have a known response to those
energies.
There are a number of instruments and detectors that have been developed that meet these
criteria, and they’ll be briefly described here. One commonly used instrument that’s found in
accelerator environments is the REM counter, shown below
Typical REM counter/monitor used at accelerators.
The REM counter works by surrounding a detector that is very sensitive to thermal neutrons
with a large amount of moderating material, such as polyethylene. The poly will moderate (slow
down) fast neutrons to thermal energies, where they are then detected by the thermal neutron
detector placed in the center. The next figure shows a schematic of a typical REM counter.
Neutron properties and definitions (supplement). Heilbronn L. https://three.jsc.nasa.gov/articles/Heilbronn_Neutron_Supplement.pdf. Date posted: 07-09-2015.
Cut away view of a REM counter. The 3He detector in the center is very sensitive to thermal
neutrons
The REM counter has a real time response, will not respond to a gamma ray field up to 200
R/hr, has a uniform response to low energy neutrons (such as fission neutron energies), and
can give an equivalent dose response if given information about the incident neutron energy
spectrum. The disadvantages of the REM counter is that it responds to a limited range of
neutrons (generally below 20 MeV, although the SWENDII detectors claim responses up to 100
MeV), the equivalent dose response is not accurate in a non-standard field, and incident
charged particles can also give an unwanted response in the detector.
The fission chamber is an instrument used at high energy neutron facilities, such as the white
neutron source at Los Alamos (LANSCE). It has the advantage that is responds to a much
wider range of energies than the REM counter, is generally unresponsive to gamma rays, and is
ideal for determining the total fluence of neutrons incident on a sample. The disadvantage is
that one needs to know the neutron energy spectrum beforehand in order to determine the
neutron dose on target. Established facilities such as LANSCE have standard, well
characterized fields that they deliver, allowing for the determination of neutron dose, if needed.
The figure below shows a schematic of the fission chamber. The chamber works by depositing
a thin layer of 252Cf on one of the parallel plates. When neutrons hit Cf, they can create a fission
event that deposits a large amount of charge into the ionization chamber. The probability of
creating a fission event is uniform with neutron energy, and as such the fission chamber is used
in neutron fields containing a wide range of neutron energies, yielding information on total
neutron flux.
Neutron properties and definitions (supplement). Heilbronn L. https://three.jsc.nasa.gov/articles/Heilbronn_Neutron_Supplement.pdf. Date posted: 07-09-2015.
Fission chamber. Neutrons are incident from the top, and the 252Cf is deposited on the top
layer.
The thimble chamber (ionization chamber) that is used at NSRL and many other accelerators
can be used for neutron dosimetry. Because it has been widely used, its response to neutron
fields below 20 to 50 MeV has been well characterized. However, the detector is also sensitive
to charged particles and gamma rays, and as such may not be as useful in a neutron field that
has associated gammas or charged particles.
Assuming that the dosimetry system is in place, the radiobiologist will want to choose a facility
that can deliver the neutron energies needed for the experiment. The final section deals with
the types of neutron facilities and sources that are available.
Sources of Neutrons
1. Accelerator-based systems
Accelerators can deliver mono-energetic beams of neutrons using the (d,T) and (d,D)
interactions where a deuteron beam is delivered onto either a triton or deuteron target. The
(d,T) reaction yields 14.1 MeV neutrons, whereas the (d,D) reaction yields 2.5 MeV neutrons.
The dosimetry is greatly simplified due to the fact that the neutron just has one energy. Also,
these accelerators are capable of delivering large fluences of neutrons over a short period of
time.
Accelerators can also deliver quasi-monoenergetic beams via 7Li(p,n), 9Be(p,n), Ti(d,n)
reactions. With those reactions, most of the neutron energies reside within a small window, but
there usually is an associated flux of low energy neutrons that is also present and cannot be
avoided. Some facilities create a beam of neutrons with a large range of energies, and through
the use of timed shutters, can cut out a large dynamic range of neutron energies from the beam
and deliver a narrow window of neutron beam energies. The total fluences at these facilities is
somewhat limited, however, especially if one desires a narrow range of neutron energies.
Quasi-monoenergetic facilities include the Crocker Cyclotron (Davis, CA), TSL (Uppsala U,
Sweden), Cyric (Tohoku U., Japan), and the US Naval academy.
Neutron properties and definitions (supplement). Heilbronn L. https://three.jsc.nasa.gov/articles/Heilbronn_Neutron_Supplement.pdf. Date posted: 07-09-2015.
Facilities such as LANSCE and the Spallation Neutron Source (SNS, located at Oak Ridge
National Laboratory) are called white neutron sources because they deliver beams of neutrons
over a very wide range of energies. LANSCE will deliver neutrons from a few MeV up to 800
MeV, and the SNS has neutrons up to 1 GeV. If needed, small windows of energy can be
selected out of the white beam through shuttering and timing techniques, but in those cases the
background created by the neutrons outside the window must be dealt with either through a
second experiment that just measures the background, or through online vetoing techniques
that stop data collection when background is present.
Reactors are capable of delivering thermal neutron fluences as high as 1015 neutrons per
second per cm2. Thus, dose rates are high, albeit at thermal energies, which may not be a
great concern in space. Also, reactor neutron beams have relatively high gamma ray
backgrounds that must be taken into account with the dosimetry.
Radioactive neutron sources such as PuBe, AmBe, SbBe, 252Cf deliver a “white” beam of fission
energy neutrons, typically up to 8 or 10 MeV with a peak around 3 MeV (see figure below). The
advantages of these sources is that they represent the neutron energies in most terrestrial
environments where sources are present, and these sources are portable and can be used
most any place that has a Radiation Safety Officer. The disadvantages to using these sources
is that the fluences are very low, requiring long run times in order to collect enough dose.
These sources also have an appreciable gamma ray background that must be taken into
account.
Relative neutron yield as a function of neutron energy from a AmBe source.
Neutron properties and definitions (supplement). Heilbronn L. https://three.jsc.nasa.gov/articles/Heilbronn_Neutron_Supplement.pdf. Date posted: 07-09-2015.